The object of the present invention is to provide a method for optimizing the triggering of an array of marine seismic sources and a shooting control device for implementing the method.
Conventional marine seismic prospecting methods are usually carried out by using a wave emission array and a wave reception array towed by a ship along a seismic profiling plane which is to be studied. The waves generated by the emission array are reflected by different reflectors of immersed formations and are received by the reception array, which generally consists of a seismic streamer of great length along which a large number of sensors are arranged.
The emission array consist of a source, and more commonly, of a plurality of sources towed while immersed and connected with the ship by groups of multifunction cables or umbilicals. Impulse sources are used most often. The form of the produced wave depends on the type of source. If the sources are of the explosion type, such as airguns for example, the main peak is produced first. With sources of the implosion type such as waterguns, the main peak is preceded by a precursory peak with a lower amplitude.
These sources are immersed, as the case may be, at substantially equal depths or systematically at different depths. The triggering times are selected with precision in view of the particular layout chosen for the emission array in order to obtain a powerful and directional source. It is a matter of obtaining, by selecting the triggering times of the different sources, the phasing of their main respective peaks in a certain direction. The operation is generally complex because multiple parameters have to be taken into account. According to the type of source, its depth of use and its mechanical state after the number of "shootings" it has performed previously, the effective time when the main peak of the source occurs may vary within notable proportions.
A shooting sequencer, adapted to take into account the different parameters characterizing the emission array used, is generally utilized to obtain the desired phasing. Sensors are located close to the sources to determine the triggering times and/or the form of the produced impulses.
Systems where sequencers are used for controlling impulse sources are described, for example, in U.S. Pat. Nos. 4,599,712, 4,693,336, 4,718,045, 4,739,858 and in European Patent Applications Nos. 31,196 and 48,623.
The large number of parameters likely to influence the triggering delay of each one of the sources of the array makes the desired phasing of the main peaks very complex. In a general way, the devices providing the sequencing of a multi-source emission array only take into account a limited number of parameters, such as the structural triggering delay, the depth of immersion, etc., and do not take into account the fluctuations of these parameters in the real operating conditions at sea.